High-frequency intracellular transposition of a defective mammalian provirus detected by an in situ colorimetric assay

We devised an indicator gene for retrotransposition, nlsLacZ^RT, which contains the Escherichia coli lacZ gene fused to a nuclear location signal (nlsLacZ), engineered in such a way that the gene is expressed only if the structure in which it has been inserted transposes itself through an RNA intermediate. A cloned murine leukemia retrovirus with an ecotropic host range (Moloney murine leukemia virus), rendered defective by a large deletion encompassing the three viral gag, pol, and env open reading frames, was marked with this indicator gene and introduced by transfection into heterologous feline cells. No β-galactosidase activity could be detected among the clonal cell population, unless the defective provirus was complemented in trans by the gag-pol gene products. Under these conditions, cell variants which disclosed an easily detectable nuclear blue coloration upon in situ 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside staining were observed. Fluorescence-activated cell sorting of the β-galactosidase-positive cells, followed by Southern blot analysis, demonstrated an unambiguous correlation between nlsLacZ^RT activation and retrotransposition of the marked provirus. Transposition occurs at a high frequency (up to 10^-4 events per cell per generation), which is dependent on the level of expression of the gag-pol gene and is concomitant with the release of noninfectious retroviruslike particles which are the hallmarks, but not the intermediates, of the intracellular transposition process.